The present invention relates to refrigeration systems and, more particularly, relates to methods and control systems for protecting evaporators in refrigeration systems against freezeups.
Conventional refrigeration systems utilize a recirculating refrigerant for removing heat from the low temperature side of the refrigeration system and for discharging heat at the high temperature side of the refrigeration system. The work input necessary to operate the refrigeration system is provided by a motor driven compressor which receives low pressure gaseous refrigerant and compresses it to a high pressure. This high pressure gaseous refrigerant is supplied to a condenser where heat is removed from the gaseous refrigerant to condense it to a liquid. This liquid refrigerant is then supplied through an expansion valve to an evaporator wherein heat is transferred from a heat transfer fluid to the liquid refrigerant to evaporate the liquid refrigerant. The heat transfer fluid is thereby cooled and then used to cool a load, such as to cool a building. This evaporated refrigerant from the evaporator is returned to the compressor for recirculation through the refrigeration system.
Normally, the heat transfer fluid used in an evaporator of a conventional refrigeration system of the type described above is a liquid such as water. Usually, the liquid enters one end of the evaporator, is cooled as it flows through the evaporator, and then exits at another opposite end of the evaporator. It is highly desirable to maintain the heat transfer liquid flowing through the evaporator at a temperature above the freezing temperature of the heat transfer liquid. If the liquid is not maintained above its freezing temperature then the liquid may freeze in the evaporator thereby preventing proper operation of the refrigeration system and possibly damaging the evaporator. This is especially true if the heat transfer fluid is water because water increases in volume when changing state from a liquid to a solid.
The danger of the heat transfer fluid freezing in the evaporator is increased if due to some malfunction there is no flow or abnormally low flow of the heat transfer fluid through the evaporator. Therefore, flow sensors have been used to detect whether there is normal flow of heat transfer fluid through an evaporator when a refrigeration system is operating. If no flow or an abnormally low flow is detected the refrigeration system is shut down. However, these flow sensors are mechanical devices inherently subject to mechanical failure or difficulties which may provide a faulty indication of the flow through the evaporator thereby needlessly shutting down operation of the refrigeration system or possibly allowing a freezeup to occur in the evaporator. Also, these flow sensors provide no direct indication of the actual temperature of the heat transfer fluid flowing through the evaporator relative to the freezing temperature of the heat transfer fluid. Therefore, another protection device for sensing the actual temperature of the heat transfer fluid must be provided in addition to the flow sensor.